An image forming apparatus such as a printer, copying machine, or facsimile apparatus or an image forming apparatus used as an output device for a composite electronic device or workstation such as a computer or wordprocessor is designed to print desired information such as characters and images on a printing medium such as a paper sheet on the basis of image information.
Such image forming apparatuses can be classified into the ink-jet type, wire-dot type, thermal type, laser beam type, and the like.
For example, an ink-jet printer using an ink-jet type printhead is designed to print by discharging ink from the printhead onto a printing medium, and has the following advantages.
This printer allows easy miniaturization of the printhead and can print a high-resolution image at high speed. In addition, the printer can print an image on plain paper without requiring any special treatment, operates at a low running cost, and produces low noise because of the non-impact type. Furthermore, the printer can easily print a color image by using multiple color inks.
In a serial printer designed to complete an image by performing scanning a plural number of times, i.e., discharging ink onto a printing medium by reciprocally scanning such an ink-jet type printhead, density irregularity may occur due to the influence of convey precision or streaks appear on a printed image due to variations in the discharging operation of the printhead, resulting in a deterioration in print quality, depending on the convey precision of a printing medium such as a printing paper sheet or the discharging performance of the printhead.
In order to solve such a problem, a multipass printing scheme has been proposed, in which the same partial printing area on a printing medium is scanned by different areas of a printhead a plural number of times to complete printing on the partial printing area.
In this multipass printing scheme, a mask pattern with a predetermined size is applied (ANDing) to original image data to decrease the number of pixels to be printed by one scanning operation by the printhead, and different printing elements (nozzles) of the printhead are used to discharge ink in each scanning operation. In addition, printing is performed with the unit of transfer of a printing medium being set to be shorter than the printing width of the printhead. This makes it possible to suppress density irregularity and the generation of streaks due to the concentration of variations in discharge performance among the printing elements of the printhead on a specific place.
As such a conventional technique, a method using a mask matrix having a random mask pattern distribution in an area having a predetermined size is disclosed in Japanese Patent Laid-Open No. 7-52390. A printing scheme is also disclosed in Japanese Patent Laid-Open No. 10-235852, which uses a mask pattern in which at least one of the resolutions in the main scanning direction which is the scanning direction of a printhead and the sub-scanning direction which is the convey direction of a printing medium is lower than the print resolution. In the multipass printing scheme using such a random mask matrix, since the discharge period becomes irregular in the main/sub-scanning direction, density irregularity and the generation of streaks are prevented. It is also known that since the mask pattern is random, the possibility that print data may tune with the mask pattern can be made very low.
Assume that an image is to be printed on a printing medium at a resolution higher than the spacing intervals of the printing elements on the printhead which are arrayed in the sub-scanning direction. In this case, in general, a scheme of printing an image by multipass printing upon setting the driving resolution of the convey amount of a printing medium in the sub-scanning direction to the desired resolution.
If, however, the driving resolution of the printing medium convey mechanism is set in accordance with the print resolution in the sub-scanning direction as described above, high-precision parts need to be used as mechanical parts such as gears constituting the printing medium convey mechanism, resulting in an increase in the cost of the printing medium convey mechanism. In order to increase the driving resolution in the sub-scanning direction, the drive amount of a constituent element such as a motor of the printing medium convey mechanism which corresponds to the convey amount of printing medium must be increased. As a method of solving this problem, a method of setting the diving resolution of the printing medium convey mechanism to be lower than the resolution in the sub-scanning direction of the printing medium contrary to the former method is available.
Assume that the resolution of printing element in the sub-scanning direction is 600 dpi, and the driving resolution of the printing medium convey mechanism is set to 400 dpi. With the use of this printing medium convey mechanism, a printing medium is conveyed by 1/400 inch by a convey driving pulse with an amount of 1. That is, if 600 dpi is regarded to correspond to one pixel, a printing medium can be conveyed by a convey amount corresponding to 1.5 pixels by a convey driving pulse with an amount of 1. If, therefore, multipass printing is performed with a convey driving pulse with an amount of 2n+1, printing can be done on the printing medium at a resolution of 1,200 dpi in the sub-scanning direction. In addition, since a printing medium can be conveyed by a convey amount corresponding to three pixels by using a convey driving pulse with an amount of 2n, a print resolution of 600 dpi can be achieved in the sub-scanning direction.
With this arrangement, an increase in the cost of a printing medium convey mechanism and a reduction in throughput can be suppressed. In order to perform multipass printing by using a printhead constituted by printing elements with a resolution of 600 dpi in the sub-scanning direction, interpolation printing must be done to perform multipass printing with a multiple of three pixels in the sub-scanning direction. In addition, although the conventional mask pattern technique is useful in printing out an image with little density irregularity and few streaks by performing multipass printing, the number of mask patterns in the sub-scanning direction needs to be equal to at least 8n or 16n pixels or constituted by 2n pixels. This makes it difficult to apply this technique to a printing apparatus having the above relationship between printing elements and the driving resolution of a printing medium convey mechanism.